4 - Epithelial Cells & Specialisations Flashcards
What does a mucus membrane line?
Internal tubes that open to the exterior (e.g. GI, respiratory and urinary tracts)
What are serous membranes?
Thin, two-part membranes which line closed cavities and envelope the viscera. E.g. peritoneum, pleura, pericardium
What do serous membranes secrete and what does this do?
Watery lubricating fluid
Promotes friction-free movement of the structures they surround.
What layers does a serous membrane consist of?
Simple squamous epithelium - secretes lubricating fluid
Thin layer of connective tissue - attaches epithelium to adjacent tissues
How do serous membranes develop around organs?
- During embryogenesis, the heart, lungs and gut invaginate into a bag-like cavity
- They become surrounded by a membrane with an inner (visceral) and outer (parietal) layer, with fluid inside
The epithelial lining of blood vessels and lymphatic is called …..
Endothelium
What are the different types of simple epithelial cell?
- Squamous
- Cuboidal
- Columnar
- Pseudostratified
What are the different types of stratified epithelia?
- Squamous
- Cuboidal
- Columnar
- Transitional
What are the functions of simple squamous epithelia?
Give some examples of it’s location.
- Fast material exchange
- Barrier to fluids
- Tissue lubrication
E.g. vasculature, bowman’s capsule, serosa of lungs, heart and viscera
What are the functions of simple cuboidal epithelium?
Give some examples of their location.
- Absorption and conduit
- Absorption and secretion
- Barrier/covering
- Hormone synthesis and storage
E.g. pancreatic duct, kidney collecting duct, thyroid gland follicles
What role does simple cuboidal epithelium play in the thyroid gland?
- Lines the thyroid follicles
- Synthesises thyroglobulin and stores it outside the cell in the centre of the follicle (colloid) - only organ that does this!
- Reprocesses thyroglobulin to thyroxine, which is released into capillaries
In simple columnar epithelium, where is the nucleus positioned?
Close to the basal lamina
What are the functions of simple columnar epithelium?
Give examples of where this can be found.
- Absorption
- Secretion
- Lubrication
E.g. Stomach and gastric glands, small intestine, colon and gallbladder
In the crypts of Lieberkühn, what are the white ‘holes’ in the epithelium seen on histology? What do they do?
Goblet cells
Secrete mucus (mucins and water)
What structure can sometimes be found on simple columnar epithelium to increase surface area?
By what method can this be seen?
Microvilli
Only visible under electron microscope
The villi of the small intestine contain ………. veins, a type of lymphatic vessel to draw liquid from the gut
Lacteal
What is pseudostratified epithelium?
- All cells make contact with the basement membrane
- Not all reach the apical surface
- Nuclei lie at different levels, giving the appearance of multiple layers
What are the functions of pseudostratified epithelium?
Give examples of where this is found.
- Secretion and conduit
- Mucus secretion
- Particle trapping and removal
E.g. Nasal cavity, trachea, bronchi, ductus deferens
What is stratified squamous epithelium and what does it do?
- Multiple layers of cells, the outermost layer is thin squamous cells
- Can be non-keratinised or keratinised
Functions:
- Protection from abrasion
- Reduces water loss but remains moist
Give some examples of where stratified squamous (non-keratinised) epithelium is found and why.
The vagina - prevents abrasion and reduces water loss. Cells are also rich in glycogen, metabolised by lactobacilli which make lactic acid (low pH for destroying pathogens)
The oesophagus - prevents abrasion and keeps lining moist
Also found in oral cavity, anal canal, surface of corneas and inner side of eyelids
What is keratinised stratified squamous epithelium?
What is the outermost layer called?
- Stratified squamous epithelium where the outermost cells have lost their nuclei and become cornified (filled with keratin)
- The outermost layer is the stratum corneum
What are the functions of stratified squamous keratinised epithelium?
- Protect against abrasion and trauma
- Prevent water loss
- Prevent microbial ingress
- Shielding from UV damage
What is the primary site of stratified squamous keratinised epithelium?
The epidermis of the skin
What is the structure of the skin dermis?
- Dense, irregular, connective tissue
- Contains collagen and elastin in all directions
Which layer of the skin is thicker in thick skin (e.g. soles of feet)?
- More keratinised epidermis - as it is more likely to be abraded
- Living epidermal cells and the dermis are the same
Epithelial cells in stratified squamous keratinised epithelium are called …….
Keratinocytes
Describe the process of keratinocyte division and migration.
- Keratinocyte mitosis occurs in the basal layer of the epidermis
- Daughter cells move towards the surface, differentiate and lose ability to divide
- Cells synthesise keratins which contribute strength to the epidermis
- Normal transit time from basal -> stratum corneum is 28-40 days
The transit time for keratinocytes to reach the stratum corneum is reduced in certain diseases.
Give an example of one of these diseases and the change seen in transit time.
Psoriasis
Transit time reduced to 2-4 days (normally 28-40)
Stratum corneum is produced in abundance as silvery scales
What are melanocytes and what do they do?
- In the basal layer of epidermis
- Produce melanin - pigment of skin colour
- Sends dendritic processes into the upper layers of the skin and can transfer pigment into the keratinocytes
- Can produce more/less melanin but everyone has the same number of melanocytes
What are langerhans cells (in epidermis) and what do they do?
- Highly specialised antigen-presenting cells (present to T-lymphocytes)
- Mediate immune reactions (e.g. allergic contact dermatitis)
What is transitional epithelium and where is it found?
- Cells vary in shape (columnar, cuboidal or squamous)
- In tissues with distensibility (relaxed -> stretched)
Urinary epithelium (urothelium)
What are the functions of transitional epithelium?
- Distensibility
- Protection of underlying tissue from toxic chemicals
What are goblet cells?
- Specialised cells in epithelia that release mucins via exocytosis
- Water released by release of ions
- Have microvilli to increase surface area
What causes cystic fibrosis?
How are tissues of the body affected?
- Mutation in the CFTR gene (a chloride channel responsible for the movement of water that thins the mucus)
- Clogging and infection of the airways, breathing obstruction
- Blocked bile duct, pancreatic duct and reproductive tracts
- Thick, non-motile stools in small intestine
- Salty sweat and crystals on the skin
What are club (clara) cells?
- Found on the airway side of terminal bronchioles
- Protect the epithelium with secretory proteins and secrete solutions similar to pulmonary surfactant (breaks surface tension for gas exchange)
- Detoxify substances (CytP450 enzymes in SER)
- Also act as stem cells to regenerate respiratory epithelium
What are microfold cells?
- Found in the small intestine close to lymphatic nodules
- Trap pathogens and present to underlying dendritic cells, lymphocytes and macrophages
- Also a weak point for the epithelium, pathogens can exploit as a point of entry
What happens in the early stages of smoking damage to the lungs?
- Normal mucus layer thickens
2. Cilia die off (regeneration of cilia takes 2-4 days)
What happens in the chronic stages of smoking damage to the lungs?
- Goblet cells and basal cells proliferate
- Club cells die
- Carcinogens induce mutations and malignancy
- Pneumocytes in alveoli die
What is acute bronchitis?
- Cough and mucus production
- Breathlessness (< 3 months)
- Due to inflammation, swelling and narrowing of airways and excess mucus
- Increased risk of respiratory diseases
What is chronic bronchitis?
- Cough and mucus production
- Chronic inflammation (> 3 months)
- Inflammation, swelling and narrowing of airways
- Excess mucus
- Irreparable damage to bronchioles and alveoli
What is emphysema?
- Shortness of breath due to permanent widening of the alveoli without fibrosis
- Damage to the air sacs, loss of elastic recoil and permanent change to size of alveoli
What is COPD?
Chronic Obstructive Pulmonary Disease (COPD)
Emphysema and chronic bronchitis
What is asthma?
- wheeze, shortness of breath, chest tightness, cough
- variable expiratory airflow limitation
- caused by bronchospasm (tightens smooth muscle layer)
- obstruction caused by mucus and narrowing of conducting airways
- often has a trigger (e.g. cold, exercise, allergens, stress)
